REPRODUCTION IN ANIMALS

EPRODUCTION IN ANIMALS

o you recall the processes of

digestion, circulation and

respiration which you have

studied in your previous classes? These

processes are essential for the survival

of every individual. You have also learnt

about the process of reproduction in

plants. Reproduction is essential for the

continuation of a species. Imagine what

would have happened if organisms had

not reproduced. You will realise that

reproduction is very important as it

ensures the continuation of similar

kinds of individuals, generation after

generation.

You have already learnt in your

previous class about reproduction in

plants. In this chapter, we shall learn

how reproduction takes place in

animals.

6.1 Modes of Reproduction

Have you seen the young ones

of different animals? Try to name some

of the young ones by completing

Table 6.1 shown in examples at S. No.

1 and 5.

You must have seen the young

ones of various animals being born.

Can you tell how chicks and

caterpillars are born? How are kittens

and puppies born? Do you think that

these young ones looked the same

before they were born as they do now?

Let us find out.

Just as in plants, there are two modes

by which animals reproduce. These are:

(i) Sexual reproduction, and

(ii) Asexual reproduction.

6.2 Sexual Reproduction

Try to recall reproduction in plants

which you studied in Class VII. You will

remember that plants that reproduce

sexually have male and female

reproductive parts. Can you name these

parts? In animals also, males and

females have different reproductive parts

or organs. Like plants, the reproductive

parts in animals also produce gametes

that fuse to form a zygote. It is the zygote

which develops into a new individual.

This type of reproduction beginning

from the fusion of male and female

gametes is called sexual reproduction.

Let us find out the reproductive parts

Table 6.1

S. No. Animal Young one

1. Human Baby

2. Cat

3. Dog

4. Butterfly

5. Hen Chick

6. Cow

7. Frog

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in humans and study the process of

reproduction in them.

Male Reproductive Organs

The male reproductive organs include a

pair of testes (singular, testis), two sperm

ducts and a penis (Fig. 6.1). The testes

produce the male gametes called

sperms. Millions of sperms are produced

by the testes. Look at Fig. 6.2 which

shows the picture of a sperm. Though

sperms are very small in size, each has

a head, a middle piece and a tail. Does

it appear to be a single cell? Indeed, each

sperm is a single cell with all the usual

cell components.

Fig. 6.1: Male reproductive

organs in humans

What purpose does the

tail in a sperm serve?

Fig. 6.3 : Female reproductive organs in

humans

Female Reproductive Organs

The female reproductive organs are a pair

of ovaries, oviducts (fallopian tubes) and

the uterus (Fig. 6.3). The ovary produces

Oviduct

Ovary

Uterus

Head

Middle piece

Tail

Fig. 6.2 : Human sperm

REPRODUCTION IN ANIMALS

Sperm

duct

Testis

Penis

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Boojho recalls that the size

of eggs in animals varies.

The egg may be very small

as in humans, much larger

as in hens. Ostrich egg is

the largest!

Fertilisation

The first step in the process of

reproduction is the fusion of a sperm

and an ovum. When sperms come in

contact with an egg, one of the sperms

may fuse with the egg. Such fusion of

the egg and the sperm is called

fertilisation (Fig. 6.5). During

fertilisation, the nuclei of the sperm and

the egg fuse to form a single nucleus.

This results in the formation of a

fertilised egg or zygote (Fig. 6.6). Did

Fig. 6.5 : Fertilisation

Fig. 6.4 : Human Ovum

Nucleus

Sperms

Ovum

Fusing

nuclei

The process of fertilisation is the

meeting of an egg cell from the mother

and a sperm cell from the father. So,

the new individual inherits some

characteristics from the mother and

some from the father. Look at your

brother or sister. See if you can recognise

some characters in them similar to those

of your mother or your father.

Fertilisation which takes place inside

the female body is called internal

fertilisation. Internal fertilisation

occurs in many animals including

humans, cows, dogs and hens.

Fig. 6.6 : Zygote

female gametes called ova (eggs)

(Fig. 6.4). In human beings, a single

matured egg is released into the oviduct

by one of the ovaries every month.

Uterus is the part where development of

the baby takes place. Like the sperm,

an egg is also a single cell.

you know that the zygote is the

beginning of a new individual?

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REPRODUCTION IN ANIMALS

Why do fish and

frogs lay eggs in

hundreds whereas a

hen lays only one

egg at a time?

Have you heard of

test tube babies?

Boojho and Paheli’s teacher once told

them in the class that in some women

oviducts are blocked. These women

are unable to bear babies because

sperms cannot reach the egg for

fertilisation. In such cases, doctors

collect freshly released egg and

sperms and keep them together for a

few hours for IVF or in vitro

fertilisation (fertilisation outside the

body). In case fertilisation occurs,

the zygote is allowed to develop for

about a week and then it is placed in

the mother’s uterus. Complete

development takes place in the uterus

and the baby is born like any other

baby. Babies born through this

technique are called test-tube

babies. This term is actually

misleading because babies cannot

grow in test tubes.

female lays hundreds of eggs. Unlike

hen’s egg, frog’s egg is not covered by a

shell and it is comparatively very

delicate. A layer of jelly holds the eggs

together and provides protection to the

eggs (Fig. 6.7).

You will be surprised to know that

in many animals fertilisation takes

place outside the body of the female.

In these animals, fertilisation takes

place in water. Let us find out how

this happens.

Activity 6.1

Visit some ponds or slow-flowing

streams during spring or rainy

season. Look out for clusters of

frog’s eggs floating in water.

Write down the colour and size of

the eggs.

During spring or rainy season, frogs

and toads move to ponds and slow-

flowing streams. When the male and

female come together in water, the

As the eggs are laid, the male

deposits sperms over them. Each

sperm swims randomly in water with

the help of its long tail. The sperms

come in contact with the eggs. This

results in fertilisation. This type of

fertilisation in which the fusion of a

male and a female gamete takes place

outside the body of the female is called

external fertilisation. It is very

common in aquatic animals such as

fish, starfish

, etc.

Fig. 6.7 : Eggs of frog

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Development of Embryo

Fertilisation results in the formation of

zygote which begins to develop into an

embryo [Fig. 6.8(a)]. The zygote divides

repeatedly to give rise to a ball of cells

[Fig. 6.8(b)]. The cells then begin to form

groups that develop into different tissues

and organs of the body. This developing

structure is termed an embryo. The

embryo gets embedded in the wall of the

uterus for further development

[Fig. 6.8(c)].

The embryo continues to develop in

the uterus. It gradually develops body

Fig. 6.8 : (a) Zygote formation and development of an embryo from the zygote; (b) Ball of cells

(enlarged); (c) Embedding of the embryo in the uterus (enlarged)

How could a single cell

become such a big

individual?

(b)

(a)

(c)

Uterus

Embedding

embryo

Ovary

Ovulation

Zygote

Uterus

wall

Developing

embryo

Though these animals lay

hundreds of eggs and release

millions of sperms, all the eggs

do not get fertilised and develop

into new individuals.

This is because the eggs and

sperms get exposed to water

movement, wind and rainfall.

Also, there are other animals in

the pond which may feed

on eggs. Thus, production

of large number of eggs and

sperms is necessary to

ensure fertilisation

of at least

a few of them.

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REPRODUCTION IN ANIMALS

parts such as hands, legs, head, eyes,

ears etc. The stage of the embryo in

which all the body parts can be

identified is called a foetus (Fig. 6.9).

When the development of the foetus is

complete, the mother gives birth to

the baby.

place inside the egg shell during this

period? After the chick is completely

developed it bursts open the egg shell.

In animals which undergo external

fertilisation, development of the embryo

takes place outside the female body. The

embryos continue to grow within their

egg coverings. After the embryos develop,

the eggs hatch. Y

ou must have seen

numerous tadpoles swimming in ponds

and streams.

Viviparous and Oviparous Animals

We have learnt that some animals give

birth to young ones while some animals

lay eggs which later develop into young

ones. The animals which give birth to

young ones are called viviparous

animals. Those animals which lay eggs

are called oviparous animals. The

following activity will help you

understand better and differentiate

between viviparous and oviparous

animals.

Activity 6.2

Try to observe eggs of the following

organisms – frog, lizard, butterfly

or moth, hen and crow or any other

bir

d. Were you able to observe eggs

of all of them? Make drawings of the

eggs that you have observed.

The eggs of a few animals are easy

to observe because their mothers lay

them outside their bodies. These are

examples of oviparous animals. But you

would not be able to collect the eggs of

a dog, cow or cat. This is because they

do not lay eggs. The mother gives birth

to the young ones. These are examples

of viviparous animals.

Internal fertilisation takes place in

hens also. But, do hens give birth to

babies like human beings and cows? You

know that they do not. Then, how are

chicks born? Let us find out.

Soon after fertilisation, the zygote

divides repeatedly and travels down the

oviduct. As it travels down, many

protective layers are formed around it.

The hard shell that you see in a hen’s

egg is one such protective layer.

After the hard shell is formed around

the developing embryo, the hen finally

lays the egg. The embryo takes about 3

weeks to develop into a chick. You must

have seen the hen sitting on the eggs to

provide sufficient warmth. Did you know

that development of the chick takes

Fig. 6.9 : Foetus in the uterus

Uterus

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Can you now give some more

examples of viviparous and oviparous

animals?

Young Ones to Adults

The new individuals which are born or

hatched from the eggs continue to grow

till they become adults. In some

animals, the young ones may look very

different from the adults. The life cycle

of frog is shown in Fig. 6.10.

Observe the different stages of frog

starting from the egg to the adult stage.

We find that there are three distinct

stages, that is, egg → tadpole (larva) →

adult. Don’t the tadpoles look so

different from the adults? Can you

imagine that these tadpoles would some

day become frogs?

The tadpoles transform into adults

capable of jumping and swimming. The

transformation of the larva into an adult

through drastic changes is called

metamorphosis. What about the

changes that we observe in our body as

we grow? Do you think we too undergo

metamorphosis? In human beings,

body parts similar to those present in

the adults are present from the time

of birth.

6.3 Asexual Reproduction

So far, we have learnt about reproduction

in some familiar animals. But what

about very small animals like hydra and

microscopic organisms like amoeba? Do

you know how they reproduce? Let us

find out.

Fig. 6.10 :

Life cycle of frog

(a) Eggs

(b) Early tadpole

(d) Adult frog

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REPRODUCTION IN ANIMALS

Activity 6.3

Get permanent slides of

hydra. Observe them using

hand lens or a microscope.

Look out for any bulges

from the parent body.

Count the number of

bulges that you see in

different slides. Also, note

the size of the bulges. Draw

the diagram of hydra, as

you see it. Compare it with

the Fig. 6.11.

Fig. 6.11 : Budding in Hydra

Fig. 6.12 : Binary fission in Amoeba

(a)

(b)

(c)

(d)

Dividing nucleus

Daughter amoebae

In each hydra, there may be one or

more bulges. These bulges are the

developing new individuals and they are

called buds. Recall the presence of buds

in yeast. In hydra too the new individuals

develop as outgrowths from a single

parent. This type of reproduction in

which only a single parent is involved is

called asexual reproduction. Since new

individuals develop from the buds in

hydra, this type of asexual reproduction

is called budding.

Another method of asexual

reproduction is observed in the

microscopic organism, amoeba. Let us

see how this happens.

You have already learnt about the

structure of amoeba. You will recall that

amoeba is a single-celled organism

[Fig. 6.12(a)]. It begins the process of

reproduction by the division of its

nucleus into two nuclei [Fig. 6.12(b)].

This is followed by division of its body

into two, each part receiving a nucleus

[Fig. 6.12(c)]. Finally, two amoebae are

produced from one parent amoeba

[Fig. 6.12(d)]. This type of asexual

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Story of Dolly, the Clone

Cloning is the production of an exact copy of a cell, any other living part, or a

complete organism. Cloning of an animal was successfully performed for the first

time by Ian Wilmut and his colleagues at the Roslin Institute in Edinburgh,

Scotland. They successfully cloned a sheep named Dolly [Fig. 6.13 (c)]. Dolly was

born on 5

July 1996 and was the first mammal to be cloned.

(a) Finn Dorsett sheep (b) Scottish blackface ewe (c) Dolly

Fig. 6.13

During the process of cloning Dolly, a cell was collected from the mammary gland

of a female Finn Dorsett sheep [Fig. 6.13 (a)]. Simultaneously, an egg was obtained

from a Scottish blackface ewe [Fig. 6.13 (b)]. The nucleus was removed from the

egg. Then, the nucleus of the mammary gland cell from the Finn Dorsett sheep

was inserted into the egg of the Scottish blackface ewe whose nucleus had been

removed. The egg thus produced was implanted into the Scottish blackface ewe.

Development of this egg followed normally and finally Dolly was born. Though

Dolly was given birth by the Scottish blackface ewe, it was found to be absolutely

identical to the Finn Dorsett sheep from which the nucleus was taken. Since the

nucleus from the egg of the Scottish blackface ewe was removed, Dolly did not

show any character of the Scottish blackface ewe. Dolly was a healthy clone of

the Finn Dorsett sheep and produced several offspring of her own through normal

sexual means. Unfortunately, Dolly died on 14

February 2003 due to a certain

lung disease.

Since Dolly, several attempts have been made to produce cloned mammals.

However, many die before birth or die soon after birth. The cloned animals are

many-a-times found to be born with severe abnormalities.

reproduction in which an animal

reproduces by dividing into two

individuals is called binary fission.

Apart from budding and binary fission,

there are other methods by which a

single parent reproduces young ones.

You will study about these in your

higher classes.

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